The present invention relates to a field-effect transistor and fabrication method thereof and an image display apparatus using them. More particularly, the present invention deals with an amorphous silicon thin film transistor (TFT) for active matrix liquid crystal displays.
Active matrix liquid crystal displays (AMLCD) have the following features, namely, thin shape, light weight, low power consumption and high quality display, and have been produced in large quantities recently.
AMLCD usually use amorphous silicon thin film transistors as switching devices.
A staggered-type structure, which consists of source/drain electrodes, a gate electrode, a channel active layer with transistor behavior, and so on, is used popularly for amorphous silicon thin film transistors.
There are two types of amorphous silicon thin film transistors of staggered structure. One is an inverted staggered-type, in which the gate electrode and the glass substrate are located on the same side with respect to the channel active layer. The other type is a staggered type, in which the gate electrode and the glass substrate are located on opposite sides with respect to the channel active layer.
Regarding the amorphous silicon thin film transistor of the staggered type, the thin film transistor structure is constructed such that the source/drain electrodes are made from transparent electrode material. Moreover, the thin film transistor is fabricated using discharging impurity gas so that source/drain areas can be formed. An amorphous silicon layer including impurity is not used in the fabrication process. Details are found in Japanese Unexamined Patent Publication(Kokai) No. Sho-62-81064.
The basic structure of the amorphous silicon thin film transistor described in the above stated Publication, is shown in FIG. 1.
Referring to FIG. 1, the amorphous silicon thin film transistor described above consists of an insulation substrate 1, a pair of source and drain electrodes made of transparent conductive film 2, a semiconductor film 4, a gate insulation film 5, and a gate metal film 6.
Transparent electrode material such as ITO(indium-tin-oxide) generally has high resistivity compared to metallic material such as aluminum or chrome.
However, the long signal wirings laid vertically, and the long gate wirings laid horizontally, on the active matrix liquid crystal display need to have low resistance.
Therefore, thin film transistor such as that shown in FIG. 1 that utilizes transparent electrode material such as ITO for the signal wiring has the problem that signal wiring resistance is high and cannot be reduced.
A solution to this problem would be to add a metal film 3 to the source/drain transparent electrode of the thin film transistor shown in FIG. 1. Adding the metal film 3 as a signal line as shown in FIG. 2 reduces resistance and the structure shown in FIG. 2 is already being used in practice.
In the case of the thin film transistor structure shown in FIG. 2, in which a low resistance signal line made of a metal such as chrome is used for the source/drain electrode, the metal film 3 and the adjacent transparent picture element electrode must be insulated electrically.
Because the transparent picture element electrode and the metal film 3 are formed by two different photoresist processes, alignment precision tolerance between the two photoresist processes, etching process precision tolerance, and minimum isolation spacing width are necessary. Therefore, when the thin film transistor and the transparent picture element electrode are placed two-dimensionally in a display equipment using the thin film transistor structure shown in FIG. 2, the spacing width between the metal film 2 and the adjacent transparent picture element electrode must be wider than if the thin film transistor structure of FIG. 1 were used.
As a result, when the display equipment is made of the thin film transistor structure shown in FIG. 1 or in FIG. 2, the transparent picture element area of the thin film transistor structure shown in FIG. 2 must be smaller than that of the thin film transistor structure shown in FIG. 1 of the same size. That is, there would arise the problem that the use of the thin film transistor structure in FIG. 2 would result in a decrease in the aperture efficiency of the liquid crystal display.
Further, regarding TFT-LCD (thin film transistor liquid crystal display), the gate electrode of the thin film transistor is connected to a so-called scan line, the source electrode is connected to, for example, a data line, and the drain electrode is connected to the picture element electrode. It will be noted that, in the case of data writing for instance, a charge signal is transferred from the source electrode to the drain electrode while a discharge signal is transferred in the opposite direction. Since we could not exactly define which electrode is the source electrode and which electrode is the drain electrode of the thin film transistor, we call one of the electrodes a source/drain electrode, and a pair of electrodes, source/drain electrodes.